Filter cloth and replaceable filter module

ABSTRACT

A filter cloth whose underside comprises substantially parallel, additional yarns  8  that are thicker than the rest of the yarns of the cloth, substantially parallel channels  9  being formed between the yarns, wherein filtered liquid passed through the cloth is allowed to flow in the direction of the surface of a filtering element  10  between the filtering portion of the cloth and the surface of the element. The invention further relates to a filtering module manufactured from the filter cloth of the invention.

This is a Division of application Ser. No. 09/986,422 filed Nov. 8, 2001now U.S. Pat. No. 6,719,148, which in turn is a Continuation ofInternational Application PCT/Fl00/00413 filed May 9, 2000 whichdesignated the U.S. and was published under PCT Article 21(2) inEnglish. The entire disclosure of the prior applications is herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to filter cloth composed of a plurality of yarnsin the transverse and the longitudinal directions, the filter clothcomprising a filtering portion having a structure and density accordingto desired filtering characteristics for separating liquid from amixture consisting of solids and liquid, and which filter cloth isfurther to be arranged against a filtering element in a filteringapparatus, and the structure of the underside of the filter cloth, i.e.a portion facing the filtering element, comprises thicker yarns than aportion facing the upper side of the cloth for the flow of liquidfiltered by the cloth.

The invention further relates to a filtering module to be arranged on afiltering element as a filtering surface when liquid is separated from amixture consisting of solids and liquid by means of a filteringapparatus, which filtering module is made of filter cloth comprising afiltering layer composed of yarns in the transverse and the longitudinaldirections, thicker yarns than the rest of the yarns of the cloth havingbeen arranged while manufacturing the cloth in the bottom of the filtercloth for the flow of liquid.

BACKGROUND OF THE INVENTION

Different machine screens, such as disc filters and drum filters, areused for removing liquid from a mixture consisting of liquid and drysolids. A disc filter typically comprises a plurality of triangularfilter segments arranged in a rotating frame onto which segments arearranged filter bags made of filter cloth. The liquid in the mixture tobe filtered is allowed to penetrate through the filter cloth and furtherinto a segment through openings extending to the surface of the filtersegment. In addition to the segments, depending on the structure of thefilter, different box-like and cylinder-like pieces and other elementsare used onto which a bag or a similar filtering module made of filtercloth is stretched by means of different mechanical tightening parts or,nowadays increasingly often, by thermal contraction. It has been foundout that if compact filter cloth is in use allowed to be pressed tightlyagainst the surface of the filtering element, the flow of the passedliquid is hindered between the filtering cloth and the element to theopenings of the element. In order to prevent this problem, a separate“additional wire bag” has been introduced between the actual filteringbag and the filtering element. The backing wire bag is made of a netliketexture forming a loose, highly permeable layer in the transversedirection of the surface of the cloth between the compact filtering bagand the surface of the element. Although such a separate backing wirebag does work relatively well in practice, fitting the backing wire bagis, however, an additional stage of operation. Furthermore, themanufacturing also causes additional costs.

SE 431 826 discloses filter cloth for separating solid particles andliquid, comprising at least two interwoven layers: a filtering layer anda supporting layer. In the supporting layer, i.e. in the portion facingthe filtering element, thicker yarns are to be used in order to achievea more coarse bottom. A drawback of the disclosed solution is, however,that the liquid that has already passed the filtering layer in thedirection of the surface of the filtering element flows inside thesupporting texture of the filter cloth. As normally, the texturecomprises yarns travelling crosswise and having different directions,whereby the yarns form obstacles to an efficient flow of liquid.

SUMMARY OF THE INVENTION

An object of the present invention is to provide filter cloth and afiltering module manufactured from the same which avoid drawbacks of theknown solutions.

The filter cloth of the invention is characterized in that the undersideof the filter cloth comprises substantially parallel yarns that arethicker than the rest of the yarns of the cloth, and that the thickeryarns form parallel channels therebetween in order to enable thefiltered liquid to flow in the direction of the surface of the filteringelement between the filtering portion of the filter cloth and thefiltering element.

The filtering module of the invention is further characterized in thatthe filtering module is manufactured from filter cloth whose underside,i.e. the surface to be against the filtering element, comprisessubstantially parallel yarns that are thicker than the rest of the yarnsof the cloth, and that channels are formed between the thicker yarns, inwhich channels the liquid filtered by the cloth is allowed to flow inthe direction of the surface of the filtering element.

The idea underlying the invention is that the filter cloth is producedsuch that while weaving the cloth, parallel, thicker yarns are arrangedon the underside of the cloth, i.e. the surface facing the filteringelement, the yarns forming therebetween parallel channels on theunderside of the cloth to enable the filtered liquid to flow in thedirection of the surface of the filtering element. The parallel, thickeryarns are additional yarns in regard to the rest of the structure of thefilter cloth; hence, they are irrelevant to the rest of the structure ofthe cloth. Furthermore, the yarns used in the cloth that travel at thethicker yarns and have the same direction as the thicker yarns arepreferably multifilaments, whereby they set tightly in the cloth in thevicinity of the monofilament, thicker yarns. Hence, the thicker yarns donot impair the density of the cloth. Furthermore, the filter cloth canbe directed in the filtering module such that the flow channels on theunderside of the cloth are located according to the positions of theopenings, i.e. the channels direct the liquid to the openings. Owing tothe thick yarns, the dense, filtering texture portion of the filteringmodule is located at a distance from the surface of the filteringelement.

An advantage of the invention is that by manufacturing filter bags to befitted onto different filtering elements and similar replaceablefiltering modules from the filter cloth of the invention, the use ofseparate backing wire bags is avoided. Hence, it is easier and quickerto change the filter bags, and no further costs result from anadditional bag. Furthermore, even thin filter cloth can be made stiffthanks to the thick yarns in the bottom of the cloth. A stiff filteringmodule stays better in place and thus lasts longer in use. Furthermore,the thick yarns in the bottom protect the actual filtering layer of thecloth, whereby sharp edges or the like of the filtering element cannotdamage the cloth. In regard to the SE publication mentioned in thedescription of the prior art, the filter cloth of the invention can bemore readily cleaned during use since dirt does not become clogged up inthe open channels in the bottom of the cloth of the invention in such amanner as dirt can be clogged in different texture structures. Asubstantial advantage is that now the filtered liquid does not flowinside the texture structure but in the flow channels formed for thepurpose, wherein there are no yarns in the transverse direction of thechannels to block the flow. Furthermore, the flow of liquid can bedirected in a desired manner when the filtering module is manufacturedsuch that the parallel flow channels in the bottom of the filter clothare arranged according to the positions of the openings and theoperation of the filtering element. It is then possible to arrange theopenings of the filtering element and the channels of the filter clothin such a manner that liquid is allowed to enter the openings of thefiltering element and further on as efficiently as possible. All in all,by using the filter cloth of the invention, the working costs of thefiltering apparatus can be reduced and the filtering efficiencyimproved.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described in closer detail in the followingdrawings, in which

FIG. 1 schematically shows the principle of a filtering apparatuswherein a filtering module manufactured from filter cloth of theinvention can preferably be used,

FIG. 2 schematically shows a structure of filter cloth of the invention,and

FIG. 3 schematically shows a filtering module made of filter cloth ofthe invention, fitted onto a filtering element.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified view of the principle of a disc filter 2comprising a plurality of filtering segments 1 a to 1 h. The disc of thefilter rotates in a basin (not shown) containing liquid and solids,whereby the liquid in the mixture passes through the filter clothstretched over the segments. The solids then remain on the outer surfaceof the filter cloth, wherefrom the solids are removed by means of doctorblades, water jets or some such washing apparatus. The liquid that haspassed the filter cloth flows to openings 3 provided in a filteringsegment and therethrough further into the filtering element, from whichit is directed forward via a duct 4. For the sake of clarity, theopenings 3 are depicted highly enlarged in the figure. For the samereason, the other segments 1 b to 1 h are indicated in dotted lines andtotally without openings. As can be seen from the figure, the openingshave the same direction as the central axle of the segment but, ofcourse, the openings can be arranged into lines having a differentdirection, or, alternatively, the entire filtering surfaces of thesegment may be evenly provided with openings. When the openings arelocated according to the figure and the filtering module is formedaccording to the line formed by the openings, the flow channels formedby the thick yarns on the underside of the module have the samedirection as the openings. For illustrative reasons, the direction ofthe thick yarns in the bottom of the module has been indicated in dottedlines 5 in the figure. When the openings and the flow channels areformed in this manner, the liquid that has passed through the clothflows efficiently to the openings and further into the segment becauseof the rotation of the filtering apparatus and gravity.

In a highly simplified manner, FIG. 2 shows a cross-sectional structureof filter cloth of the invention. The filter cloth according to thefigure is single-layer filter cloth comprising warp yarns 6 and weftyarns 7. Furthermore, thicker yarns 8 having the direction of the wefthave been woven on the surface of the bottom of the cloth. The thickeryarns are additional yarns, i.e. each thicker yarn has been arrangedparallel to and against a weft yarn 7 a located at the thicker yarn. Theparallel, thicker yarns form flow channels 9 therebetween, whereby theliquid filtered by the cloth is allowed to flow via the channels anddirected by the channels to the openings 3 provided in a filter element10. The thicker yarns are monofilaments that are highly tolerant ofcompression. The warp and weft yarns, in turn, are multifilamentsbecause they allow easier formation of a dense texture. Furthermore, inthe cloth of the invention, they flatten out and become appropriatelymoulded at the thicker yarns, whereby the cloth becomes compactregardless of the thicker yarns in the bottom. This can be seen in FIG.2 by examining particularly the weft yarns 7 a located at the thickeryarns 8. Also spun yarn can be used in the filtering portion. Further,the structure may comprise other monofilament yarns than the thick yarnsin the bottom. A monofilament having the same direction as the thickyarns and improving the stiffness of the cloth has been indicated in thefigure by reference number 11. However, the parallel yarns adjacent tothe thick yarns are, due to the compactness, preferably multifilamentsor spun yarns. Thanks to the thicker yarns, the cloth is stiff even ifthe actual filtering layer were extremely thin. Thanks to a stifferstructure, the filtering module stays better in place regardless of thefiltering and cleaning forces that it is subjected to, and,consequently, lasts longer. On the other hand, a thin filtering layerdoes not become clogged as easily as a thick layer, and it can be washedmore readily. Furthermore, the thick yarns in the bottom protect theactual filtering layer e.g. in the corners of the filtering elementwherein the filter cloth is subjected to high stress while beingstretched.

FIG. 2 also shows that the thicker yarns 8 can be arranged at desiredintervals in the cloth. The number of the thicker yarns in relation tothe other parallel yarns in the bottom can be determined according tothe use and the desired characteristics, e.g. stiffness, required of thecloth.

The filtering module can be stretched over the filtering element bymeans of different mechanical stretching parts known per se or thestretching can be carried out by thermal contraction, in which case atleast some of the yarns of the filter cloth must be heat-shrinkable. Inthis connection, heat-shrinkable yarn refers to yarn used inmanufacturing filter cloth that can be shortened by heating to such anextent that the resulting change in length can be utilized in a desiredmanner in order to stretch filter bags, filter wires or the like tightlyin place. The stretching can be controlled by selecting the yarnsappropriately. Since the thicker yarns only contract substantially intheir longitudinal direction and not in relation to their diameter, thechannels, while being contracted, substantially retain their shape;consequently, the contraction does not impair the flow of liquid. On theother hand, the thicker yarns do not have to be shrinkable; it isusually enough that the transverse yarns in relation to the thickeryarns are shrinkable. Also in such a case, the channels retain theirstructure regardless of the contraction.

FIG. 3 is a sectional view of a part of a filtering module that has beenfitted in place. In this case, the filtering element 10 is a cylinder,openings 3 having been provided through the surface of the cylinder. Thethicker yarns 8 in the bottom of the cloth form channels 9 at theopenings in the manner shown in the figure. For the sake of clarity, thetwo-layer filter cloth is shown in a highly simplified manner in thefigure. It is believed in this connection that a person skilled in theart can select an appropriate bonding type, yarn material, yarn type,etc. The filter cloth can thus be a single-or multilayer texture and itcan be made of yarns having a different cross section, as long as theyarn material is durable and has dimensional stability under the plannedworking conditions. Thick yarns per se do not affect the characteristicsof the filtering layer, so in that sense they are additional yarns inthe cloth. The diameter difference between the other yarns and thethicker yarns is preferably between 1:1.4 to 1:6, whereby a sufficientlylarge channel can be provided between the filtering layer of the clothand the surface of the element as far as the flow of liquid isconcerned. A person skilled in the art can determine the difference inthickness between the multi-and monofilaments by calculating, forexample.

The drawing and the related description are only intended to illustratethe idea of the invention. In its details, the invention may vary withinthe scope of the claims. Hence, the actual filtering layer of the filtercloth can be formed by needling thickening batt into the body texture.The thickening effect can also be achieved by an appropriate coatinglayer to be arranged on the upper surface of the cloth. In principle, atleast, it is also possible to use a body formed from non-woven yarnarrangements. Further, elements known as shrink wires to be arranged ondifferent drum filters and the like can also be manufactured from thefilter cloth of the invention. Furthermore, by directing the filtercloth appropriately, desired points of the module can be made stiff bymeans of the thick yarns in the bottom. Bent transversely to the thickyarns, the cloth is stiff, but when the cloth is bent in the directionof the thick yarns, it bends readily over a section between the thickyarns. In addition, the cloth can be manufactured such that the thickeryarns in the surface structure of the bottom of the cloth have, asdistinct from what has been disclosed above, the same direction as thewarp.

1. A disc filter bag made of a solid-liquid separation filter cloth,comprising: a filter bag, made of a solid-liquid separation filtercloth, stretched over a filtering segment in a disc filter; a singlelayer solid-liquid separation filter cloth comprising multi-filamentwarp yarns and multi-filament weft yarns; thicker monofilament yarnswoven onto the surface of the filter cloth forming flow channels forfiltrate flow, wherein the thicker monofilament yarns are spaced apart,parallel to and against the weft yarns; and the filter bag having thesame shape of the filtering segment of the disc filter.
 2. The discfilter bag as claimed in claim 1, wherein the diameter differencebetween the other yarns and the thicker yarns of the filter bag is atleast 1:1.4 or more.
 3. The disc filter bag as claimed in claim 1,wherein at least some of the yarns used in the filter bag areheat-shrinkable, whereby the heat-shrinkable yarns are shortened whensubjected to heat, thus allowing the filter bag to be stretched over thefiltering segment by thermal treatment.
 4. The disc filter bag asclaimed in claim 1, wherein batt has been needled to the filteringportion of an upper surface of the filter bag, which is a surface facingaway from the filtering segment, to obtain a denser structure.
 5. A discfilter for solid-liquid separation, comprising: a plurality oftriangular filter segments arranged in a rotating frame, the segmentforming a disc; filter bags made of filter cloth arranged onto thesegments, whereby the filter bags serve as a filtering surface whenliquid is separated from a mixture consisting of solids and liquid,wherein the filter bag having the shape of the filtering segment of thedisc filter; a basin wherein the disc is rotated and which basincontains liquid and solids; a plurality of openings provided in thefilter segments allowing the liquid that has passed the filter bag toflow into the filter segment; a single layer filter cloth comprisingmulti-filament warn yarns and multi-filament weft yarns; and thickermonofilament yarns woven onto the surface of the filter cloth formingflow channels for filtrate flow, wherein the thicker monofilament yarnsare spaced apart, parallel to and against the weft yarns.
 6. A discfilter as claimed in claim 5, wherein the thicker yarns in the undersideof the filter bag are arranged in a longitudinal direction of the filtersegment essentially towards a center duct of the disc filter.
 7. Asolid-liquid separation filtering module to be arranged on a cylindricalfiltering element as a filtering surface by means of a drum filter,including: a single layer filter cloth comprising multi-filament warpyarns and multi-filament weft yarns; and thicker monofilament yarnswoven onto the surface of the filter cloth forming flow channels forfiltrate flow, wherein the thicker monofilament yarns are spaced apart,parallel to and against the weft yarns.
 8. The solid-liquid separationfiltering module as claimed in claim 7, wherein the filter cloth isarranged such that the channels in the bottom of the cloth are directedin a longitudinal direction of the cylindrical filtering element, andthe channels in the bottom of the cloth are directed such that thechannels lead the filtered liquid to openings in the cylindricalfiltering element.
 9. The solid-liquid separation filtering module asclaimed in claim 7, wherein the filtering module comprisesheat-shrinkable yarns, whereby the heat-shrinkable yarns are shortenedwhen subjected to heat, thus allowing the filtering module to bestretched over the filtering element by thermal treatment.
 10. A drumfilter for solid-liquid separation, comprising: a filtering module; afiltering element, wherein the filtering element is a cylinder includingopenings through the surface of the cylinder, wherein the filteringmodule is arranged on a filtering element as a filtering surface;wherein the filtering module is made of a single layer filter clothcomprising multi-filament warp yarns and multi-filament weft yarns; andthicker monofilament yarns woven onto the surface of the filter clothforming flow channels for filtrate flow, wherein the thickermonofilament yarns are spaced apart, parallel to and against the weftyarns.
 11. The drum filter as claimed in claim 10, wherein the filteringmodule is arranged such that the channels in the bottom of the cloth aredirected in a longitudinal direction of the cylindrical filteringelement, and the channels in the bottom of the cloth are directed suchthat the channels lead the filtered liquid to openings in thecylindrical filtering element.
 12. The drum filter as claimed in claim10, wherein the filtering module comprises heat-shrinkable yarns,whereby the heat-shrinkable yarns are shortened when subjected to heat,thus allowing the filtering module to be stretched over the cylindricalfiltering element by thermal treatment.